Without actually researching it... I think the 478, the 702, and the 638 use SAE #2 flywheel housings exclusively.

If that's the case then you can still use the SM420, but you'll be looking for a more than likely hard to find adapter clutch housing.
Would be better off switching to at least a 5 speed OD transmission, and a number of those come with SAE housings which makes that easier.

The 478 is probably not practical even if it does fit, but would be interesting.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-